The purpose of this research is to determine how radiation therapy alters the various components of solid tumors that may affect the immune system's ability to mount effective anti-tumor responses. The goal of these studies is to gain a better understanding of the basic mechanisms involved in radiation induced changes in the tumor microenvironment to allow more effective treatment strategies combining radiation and immunotherapy. The differences in these two therapy modalities make it highly likely that their combined use could have synergistic effects on tumor destruction. Radiation is highly effective at killing large numbers of tumor cells and controlling primary disease, however, it is not easily used for the treatment of dissem- inated metastatic disease. In contrast immunotherapy, because of its specificity and systemic nature, has the potential to be less toxic to normal tissues and to reach distant metastases. However, radiation therapy may itself be damaging to critical cells of the immune system, particularly in multi-radiation dose protocols in which subsequent treatments may destroy tumor infiltrating lymphocytes. We have shown that radiation induced tumor cell death can provide increased tumor antigen to stimulate immunity, and radiation can alter the tumor vasculature making it more prone to host cell infiltration. These tumor infiltrating host cells produce many cytokines, which can stimulate changes in the tumor vasculature and tumor stroma and thus alter growth potential and radiation sensitivity. For example, we have shown that interleukin-12 (IL-12), produced within the local tumor microenvironment, causes marked changes in the tumor vasculature by preventing the upregulation of vascular endothelial growth factor (VEGF) receptor 3 leading to enhanced immune cell infiltration and delayed tumor growth or rejection. In this proposal we plan to determine the mechanisms responsible for these IL-12 induced changes in tumor vasculature and in the immune cell infiltration and function, and determine the role of interferon-gamma (IFN-g) in these processes. Due to the importance of IFN-g in these IL-12 induced changes and in alterations that render tumor cells more radioresistant, we will also determine the mechanisms by which IFN-g signally mediates radioresistance and how these affect immune sensitivity of tumors. The information we obtain will be useful in improving the combination of radiotherapy and immunotherapy to more effectively treat patients with cancer.